A fan of the show "The Office" on NBC. The word stems from the fictitious paper supply company "Dunder Mifflin" featured in the show.
You can be sure a dunderhead will be plopped down in front of a T.V. at 9/8c every Thursday night.

This particular definition, although credit shown to someone else, was started years ago by Bobblehead Gary, (Gary Dunn) the greatest Office fan ever.

Dunderheads love The Office and will do everything in their power to support it.